1. Field
This invention relates to the production of semiconductor devices, and, in particular, to field-effect transistors intended for operation at microwave frequencies.
2. Prior Art
Typically, prior art field-effect transistors, commonly referred to as FETs, use aluminum as the contacting metal in the gate. Aluminum, however, is susceptible to electromigration and conversion to aluminum oxide, which increases the gate resistance and degrades RF performance.
The field-effect transistor gate also has been made of chromium, but chromium requires a diffusion barrier between the chromium and the usual gold overlayer, which adds to the cost of production.
To extend the operating frequency of a field-effect transistor into the microwave and near-millimeter-frequency ranges, it is necessary to reduce the gate length to a submicron dimension. Prior art techniques have been able to achieve gate length of the order of 0.2 micron, but in producing a gate of this length, the gate resistance was increased substantially, adversely affecting gain and noise figure.
Prior art passivation has often consisted merely of depositing a layer of silicon dioxide, despite the fact that silicon dioxide has proven not to be an effective passivation against sodium ions. In fact, silicon dioxide, when not properly processed, can act as a getter for sodium ions, resulting in degraded RF performance and reliability.
In current FET fabrication, formation of a mesa structure is one of the early steps, and this places the gate pad at a lower level than the gate, requiring the conductor from the gate pad to the gate to pass over the edge of the mesa. During the deposition process used to form the conductor, less conductor material deposits at the edge of the mesa than on the rest of the conductor path, creating a thinner, high-resistance portion at the bend in the conductor which degrades the RF performance and makes the transistor subject to burnout.
Formation of the mesa in the early steps of prior art processes has also resulted in later contamination of the mesa edge by residues left by the various subsequent metalization steps. These residues can enhance the transport mechanisms of other materials, such as gold, resulting either in a short or in degraded performance of the transistor.
Up to the present time, it has been difficult to make metal-oxide semiconductor FETs, commonly referred to as MOS FETs, of gallium arsenide because of the high surface-state density at the oxide-semiconductor interface.
MOS FETs generally offer several advantages, including low leakage, the possibility of higher carrier concentration, and superior RF performance, making the successful production of a gallium arsenide MOS FET a worthwhile, but previously unattainable goal.
Prior art methods of producing MOS FETs have suffered from a number of difficulties. Most of the currently produced MOS FETs have an ohmic contact for the drain, and the ohmic contact often has an irregular diffusion profile that can result in unreliable performance and a reduction in the ability to handle high current.
In forming ohmic contacts, nickel has been widely used as a wetting agent for germanium gold and gold, to enhance the adherence of the germanium gold to the gallium arsenide, as well as to reduce clumping. However, it has been found that the nickel also causes metal segregation.
In attempts to use nickel with gallium arsenide, an additional difficulty has been encountered: the nickel reacts with the gallium arsenide, often causing the gallium arsenide to dissociate. The result is the production of a high resistivity layer at the points of dissociation.